Launch Terminal from your Applications:Utilities folder.
Input the command top into the Terminal window then press the Enter key.
You will be displayed a list of running processes on your system. Notice each process is assigned a PID. Locate the process that is not responding and take note of its PID.
If you do not see your process on the short list drag the handle at the bottom right of the window downwards to display more of the list.
Press the q button to quit top.
Input kill followed by the PID of the process you would like to close into the Terminal window. For example, if we wanted to close uTorrent top revealed that its PID was 38209. So we would input kill 38209 then press the Enter key.
The application has now been forcefully killed and is no longer running.